A well-known phenomenon in directional drilling is that hole friction dramatically increases if a horizontal drilling segment is required. That is, static friction (drag) occurs between the mud motor, drill collars, and drill pipe, and the casing and/or open hole. This high friction is caused by the drill string bearing against the bottom side of the hole. Increases in frictional forces are also frequently observed when the drill string tool joints are pushed laterally through the hole. This static friction can cause misleading indications of weight on bit, string weight and down-hole torque making automated control of the drilling process difficult, if not impossible.
To reduce this misleading information, a drilling operator will vibrate or wiggle the drill string to cause it to slide within the hole. One way to vibrate the string is to rotate the drill string back and forth, a motion commonly referred to as oscillating the drill string. Oscillating the drill string causes the drill string to momentarily lift up in the hole thereby reducing the lateral friction. However, oscillating the drill string requires relatively rapid reversals of the drill string rotation. According to one method, such an oscillation of the drill string is done manually by the drilling operator using standard operator controls found on many conventional top drive systems. To perform the oscillation, the operator lowers the motor torque limit and rotates the drill string in a clockwise direction at a low RPM until the drill string stalls or winds-up. The direction of rotation is then changed causing the drill string to unwind and then stall or wind-up in the opposite direction. This procedure is repeated by the operator until the frictional forces are reduced.
However, this manual operation relies on the operator's skill and experience to set parameters and operate the controls correctly. Such a process is also relatively slow, and in some cases causes rapid wear on the motor brakes and drive components because of the non-automated nature of the process. Accordingly, a need exists for a horizontal drilling system having an improved and/or automated oscillation control system.